This invention relates to printing machines, and more particularly, to a toner development apparatus for developing latent electrostatic images on a charge-retentive surface and, more particularly, to the purification of the toner in the development housing of the development apparatus prior to its being presented to latent electrostatic images formed on the charge-retentive surface to thereby render the latent electrostatic image visible.
In the art of xerography or other similar image reproducing arts, a latent electrostatic image is formed on a charge-retentive surface such as a photoconductor which generally comprises a photoconductive insulating material adhered to a conductive backing. The photoconductor is first provided with a uniform charge after which it is exposed to a light image of an original document to be reproduced. The latent electrostatic images, thus formed, are rendered visible by applying any one of numerous pigmented resins specifically designed for this purpose. In the case of a reusable photoconductive surface, the pigmented resin, more commonly referred to as toner which forms the visible images, is transferred to plain paper.
It should be understood that for the purposes of the present invention, which relates to the development of latent electrostatic images with toner particles, the latent electrostatic image may be formed by means other than by the exposure of an electrostatically charged photosensitive member to a light image of an original document. For example, the latent electrostatic image may be generated from information electronically stored or generated, and the digital information may be converted to alphanumeric images by image generation electronics and optics. However, such image generation electronic and optic devices form no part of the present invention.
I have observed that paper debris in the development apparatus forms agglomerates with the toner particles or broadens the toner charge distribution. When either of the foregoing phenomena occurs excessive background development occurs. In other words, the electrostatic image on the charge-retentive surface in the non-image (i.e. background areas) attracts an undesirable quantity of toner thereto thereby causing the finished copies to have an unpleasing appearance. Moreover, with the presence of the debris in the toner, it is undesirable to recycle for reuse the residual toner removed from the charge-retentive surface.
The use of an electrostatic field in attracting toner from a charge-retentive surface has been known for some time. Arrangements for such purposes are disclosed in U.S. Pat. Nos. 3,572,923 and 3,722,018 granted to Fisher et al on Mar. 22, 1973 and Fisher on Mar. 30, 1971, respectively. The use of electrostatic attraction of toner in development systems is also well known.
The problem of not being able to recycle the residual toner for reuse has been satisfactorily solved as disclosed in U.S. Pat. No. 4,494,863 issued in the name of John R. Laing and assigned to the same assignee as the instant application. As disclosed therein, debris such as paper fibers and Kaolin are removed from the residual toner so that the toner is suitable for reuse. The foregoing is accomplished by electrostatically attracting the debris and Kaolin from a residual toner removing roll. This is effected by means of an electrically biased detoning roll which is supported contiguous the toner removing roll. I am not aware of the use of electrostatic forces for purifying toner prior to its use for developing latent electrostatic images on a charge-retentive surface wherein the purification actually takes place in the developer apparatus.